The compound phenylthiobutyl-isoquinoline of the formula ##STR2## and its preparation starting from L-serine is described, e.g., in U.S. Pat. No. 5,484,926, incorporated herein by reference. This compound is a valuable intermediate for the manufacture of pharmacologically active compounds, suitable for the treatment of viral infections, especially those caused by HIV and other retroviruses, described in U.S. Pat. No. 5,484,926, e.g., at columns 16 and 17, such as represented by formula ##STR3## wherein Ph is phenyl, and pharmaceutically acceptable salts thereof.